SCT collimation shifts

[matt]

Hi guys

I've already PM'd a couple of other 9.25 owners about this issue but I thought I'd throw it out to the wider SCT community for more feedback.

Being new to SCTs, having only owned one for a couple of months, I got bit of a surprise last night when I spent a bit of time refining my collimation.

First of all, I spent a little time getting a nice collimation on Spica which was about 35 degrees above the eastern horizon in the same place Jupiter was going to be about an hour and a half later. Sure enough, once Jupiter reached the same place in the sky the view was very nice and details were very clear, albeit affected by the atmosphere being quite low in latitude.

I then slewed to another star (sorry, don't know which one) not too far from the zenith and perfromed a star test and noticed my collimation was out. The central obstruction was not in the middle of the defocused star image but rather skewed to one side.

I slewed again to another star in the south. A different result again, but likewise revealing a collimation shift.

What's going on here? Is this the dreaded SCT mirror flop I've read so much about? Is it normal for precise collimation to be so localised, and then lost to some degree as you move the scope around the sky? Will I have to get used to making tiny adjustments in collimation if I want to move from one area of the sky to another?

I had heard/read that most SCTs hold their collimation quite nicely?

Obviously, this presents a bit of a problem from an imager's point of view

Your help very much appreciated.

[pluck]

Things to check immediately:

Is your secondary mirror / assembly loose in any way ? Sometimes the collimation screws can be tight, but the holder itself loose.

Mirror flop can be the culprit - the best way to rule out this is to use a mirror locking solution, and focus using a secondary focuser. Sometimes a regrease of the central tube (the bit the mirror cell slides up and down on) can help.

The mirror itself may be loose on its cell.

Start with the simplest things first and work down.

Good luck,

Paul

[matt]

G'day pluck

Thanks for the reply.

What do you mean by "... and focus using a secondary focuser" ?

Cheers

[pluck]

HI Matt,

Many SCT owners modify their rear cells to 'lock the primary mirror' in place and then use an after market focuser (e.g. a high quality crayford design) attached to the rear of the telescope to focus. Meade designed this into their LX200 GPS etc. scopes. Celestron haven't - although the shipping bolt holes of the C14 on mine allow some tinkering in this space.

Paul

[[1ponders] (Paul)]

Definately something funny going on Matt. I don't experience anything like that with my 8" Meade. It seems to hold collimation pretty well during the night with just the odd tweek as the ota cools and if I do a lot of slewing without the mirror lock.

Definately check to see if the secondary is a bit "loose"

[matt]

Yeah.

I'm already looking at a nice Moonlite electric focuser. I want the precise focusing you need for imaging and it would be nice to adjust focus without the associated image shift. The collimation issue is a separate one, right? Due to the primary shifting around as you move the scope around?

How do I ascertain whether the primary is loose on its cell? And what's the fix?

Paul - the 9.25 doesn't have a mirror lock so I believe it's susceptible to "slop" or moving around when you slew it around?

If the secondary is loose, how do I fix that?

[pluck]

I don't know what the c9.25 mirror/cell interface looks like, but on the C14 a large retaining screw is responsible for holding the mirror to its cell (together with RTV [silicon adhesive stuff]). I'm afraid the only way to check is disassembly, so I would leave this as a last resort - partcularly as it is the most unlikely of the three.

Check the secondary holder's rigidity to the corrector by physically trying to wobble it from behind. It should be obvious if it is loose. Also, check that the secondary mirror is fixed securely to the holder - by whatever means the C9.25 uses (probably a combination of the collimation screws and something else). You might need to take the corrector off for this - so be careful, and seek some advice on this if you are unsure.

Yes - mirror flop is an inherent design issue with SCTs. There's not much which can be done to address it other than reapplication of the damping grease used on the central tube, or a mirror lock down solution (or both).

Other C9.25 owners may have some ideas worth looking into, but in terms of generic collimation I'd check for the obvious. For example, what about your eyepiece / diagonal ? Is anything moving around back there ?

Paul

[JohnG (John)]

Matt

With any new SCT you should run the mirror fully up and down the baffle a few times to distribute the grease evenly, try this first, but description sounds like a loose collar.

JohnG

[matt]

OK. Have checked the secondary holder's rigidity to the corrector. Tried giving it a jiggle. It's rock solid. I think we can eliminate that as a possibility.

JohnG - what's the "loose collar"???

[JohnG (John)]

Have you run the focuser fully in and out, it is important to distribute the grease, try a star test before going to the next step, a lot of SCT mirror flop is corrected by this simple fix. The collar I am talking about, look into the front of the tube, where baffle and the mirror are joined there is a black ring with 2 fine holes, underneath the ring is a cork gasket, if the tube has been jossled around that ring can loosen. Check the other things first because this requires you to take the corrector off, no big deal, but you will need to mark the orientation of the corrector and the location of the shims if you do this.

JohnG

[matt]

You know what, John? I don't think I've actually done that since taking delivery of the 9.25 in mid December.

I'm going downstairs now and I'll go fully in and out on the focus numerous times and then star test tonight.

Fingers crossed that's all that's causing the problem

[syzygy]

Were you using a diagonal when you noticed this apparent loss of collimation?
If so then presumably you were rotating the diagonal to the most convenient viewing position after each slew.

Before you worry too much about the scope's internals a quick check without the diagonal might be in order.

[allan gould]

I second what SYZYGY has stated. Take out the diagonal as it can really mess with your head and collimation

[matt]

Nope. No diagonal being used.

[matt]

OK follow up.

Thought I'd try tightening up the Bob's Knobs to see if that has a positive effect.

I don't want to jinx myself but I think I see an improvement

I've tightened them up pretty tight but being a comparative newbie to SCTs I've got no idea of what is too tight or too loose.

The stage they are at now is finger tight. I don't think they're too tight. They could be tightend further but right now they're pretty firm. Obviously I don't want to tighten them to the point where it puts stress on the seconday or the corrector.

So I'd like to ask some of you more experienced SCT folk: what's the visual sign/s that you've tightened the collimation screws too tight????

How does it effect the image???

Your help much appreciated.

[Dennis]

Hi Matt

Mine are finger tight too, although I guess there could be some significant variation of what this means between different SCT operators.

If the outside and inside of focus images show smooth, circular rings with the central point well centred (you know what I mean), then I guess there are no mechanical stresses on the secondary due to over tightening.

Cheers

Dennis

[matt]

Thanks Dennis

Both inside and outside focus images showed nice circular images. Concentric rings etc

[JohnG (John)]

Matt

You didn't mention in the original post you were using Bob's Knob's. It is virtually impossible to overtighten them, the SCT's I had were kept a good finger tight, in other words, tight enough so a reasonable amount of force was required to move them, the design of the secondary is that an aluminium plate moves on a dimple and the adjustment is vide the 3 knobs. If you tighten them up so tight as to not be able to move, you will get a 3 lobed effect and you will stress the secondary.

JohnG

[matt]

Apologies for that oversight John...

but then you didn't ask!

Thanks for that extra bit of info

[JohnG (John)]

No drama's, I should have asked, would have been the first thing I would have looked at, gives the same symptoms as a loose collar. They do need to be firm.

JohnG